A magnetic head assembly typically includes write and read heads wherein the write head writes magnetic bits of information into a rotating magnetic disk in a disk drive and the read head reads the magnetic bits of information from the rotating disk. The magnetic head assembly is typically located on a slider that “flies” over a rotating magnetic disk on a cushion of air between the disk surface and the air bearing surface of the slider.
The read head typically includes a spin valve sensor and two lead layers that are connected to the side edges of the spin valve sensor through which a sense current is conducted. The spin valve sensor and leads are located between a pair of nonmagnetic electrically insulative gap layers and a pair of ferromagnetic shield layers.
In general, a spin valve sensor has a pinned ferromagnetic structure in close proximity to a free ferromagnetic layer. The direction of magnetization in the pinned ferromagnetic structure is fixed or “pinned”, in a specified direction. Some spin valve sensors use an adjoining antiferromagnetic layer to pin the direction of the pinned ferromagnetic structure and some spin valve sensors use self-pinned ferromagnetic structures, which do not use an antiferromagnetic pinning layer. A self-pinned ferromagnetic layer typically includes two ferromagnetic layers that provide large coercivity and a strong anitstropy field that obviate the antiferromagnetic pinning layer.
The direction of magnetization in the free layer is responsive to the external magnetic field to which the read element is subjected. The magnetization in the free layer is usually partially stabilized with adjacent permanent magnets. The relative directions of the magnetization in the pinned layer and the free layer determines the resistance of the read element.
In a self-pinned sensor design, the magnetic anisotropy caused by the positive magnetostriction of the pinned layers together with a compressive film stress aligns the magnetization of the pinned layers in a direction perpendicular to the air bearing surface of the slider, which is the desired state for a spin valve. The anisotropy field Hk for a given magnetostriction λ and film stress σ is given by the equation:
                              H          k                =                  3          *          λ          *                      σ            M                                              eq        .                                  ⁢        1            where M is the saturation moment of the pinned layers. To increase the pinning field Hk in the pinned layers, it is desirable to increase the compressive stress σ of the film.
The use of a metal capping layer provides a large compressive stress. For a capping layer of a thickness t and with stress σ′, the bending moment exerted by the capping layer on the underlying sensor layers, which causes a stress to develop in these layers, will be proportional to the force per unit width (F/w) of the capping layer:F/W=σ′*t  eq. 2which is proportional to the thickness of the film. Thus, one method to further increase the pinning field Hk in the pinned layers is to use a thicker capping layer. A thicker capping layer will produce larger bending moments and, consequently, more stress in the underlying sensor. Unfortunately, by increasing the thickness of a metallic capping layer, more sense current is shunted by the capping layer, which leads to a significant decrease in sensor performance. Further, significantly increasing the thickness of the capping layer is undesirable because of the increase in sensor size.
Accordingly, what is needed is an improved capping layer with a large compressive stress and high electrical resistivity.